1. Field of the Invention
The present invention relates generally to thermal fuses employing a thermosensitive pellet exploiting a characteristic in flowability of a thermosensitive material thermally deforming at an increased temperature to allow the fuse to operate precisely at a temperature, and particularly to thermal fuses employing a thermosensitive pellet using a thermosensitive material composed of a thermoplastic resin exhibiting a characteristic in flowability when it is softened or melted.
2. Description of Related Art
Thermal fuses are generally divided into two types depending on the thermosensitive material used. One is a thermal fuse employing a thermosensitive pellet using non-conductive thermosensitive material, and the other is a thermal fuse employing a low melting point fusible alloy of conductive thermosensitive material. They are both a so-called non-reset thermal switch. When its surrounding temperature increases and a prescribed temperature is reached, the fuse operates to cut off or electrically connect a current carrying path of equipment and an apparatus to protect them. The fuse operates at a temperature determined by the thermosensitive material used. Typically, it is offered commercially in products operating at a temperature ranging from 60° C. to 240° C. on a rated current ranging from 0.5 A to 15 A, and acts as an electrical protection component allowing an initial conducting or interrupt state for an initial ordinary temperature to be inverted at a predetermined operating temperature to provide an opposite interrupt or conducting state. Of the above thermal fuses, the thermal fuse employing a thermosensitive pellet is composed of a casing having opposite ends with a respective lead member attached thereto, and a pellet of a non-conductive thermosensitive material, a compression spring and a movable conductor accommodated in the casing. When a prescribed operating temperature is attained and the pellet softens or melts, the compression spring pushes and presses and thus acts on the movable conductor to move it to change a conducting or interrupt state or vice versa. The thermosensitive pellet is typically formed of a chemical agent having a prescribed melting point and formed into a prescribed geometry, granulated, made into a tablet and thus pelletized.
The thermal fuse employing a thermosensitive pellet generally employs a thermosensitive material composed of a single organic chemical compound having a known melting point, and to make it a thermosensitive pellet, binder, lubricant, pigment and the like are added to enhance granulability, provide uniform density and classify the type of the thermosensitive pellet, respectively, and the thus obtained medium is pelletized. The single organic compound includes 4methylumbelliferone, a pure chemical agent, as disclosed for example in Japanese Patent Laying-Open No. S60-138819. Furthermore, as disclosed in Japanese Patent Laying-Open No. 2002-163966 and Japanese Patent No. 2551754, two or more types of organic compounds may be mixed together to prepare and use a thermosensitive material having a different melting point. In general, a eutectic mixture is satisfactory in thermal stability and insulation stability. It is said, however, that if it is mixed with an intended chemical agent, its melting point varies. Furthermore, these chemical agents are low molecular weight compounds and are chemical agents such as certified pure reagents or other similar reagents of high purity. Furthermore, Japanese Utility Model Publication No. H6-12594 indicates a disadvantage associated with pelletization in connection with a thermosensitive pellet's insulation resistance when the pellet melts, and a resolution therefor.
Japanese Patent Laying-Open No. S50-138354 and Japanese Utility Model Laying-Open No. S51-145538 disclose a thermosensitive material composed of paraffin or a similar thermosensitive fusible substance or a heat resistant, nonconductive, synthetic resin material. However, either case is not practically used since it utilizes the melting of the thermosensitive material itself and there is a problem associated with setting an operating temperature that can be ensured, and the thermosensitive pellet's secular variation. Furthermore, Japanese Patent Laying-Open No. 2003-317589 discloses a thermal fuse employing a thermosensitive pellet that employs a thermosensitive material composed of thermoplastic resin blended with a filler. It is not easy for the thermal fuse, however, to set a highly precise and steady operating temperature.